Extensive networks of underground electrical cables are in place in many parts of the industrialized world. Such underground distribution offers great advantage over conventional overhead lines in that it is not subject to wind, ice or lightning damage and is thus viewed as a reliable means for delivering electrical power without obstructing the surrounding landscape, the latter feature being particularly appreciated in suburban and urban settings. Unfortunately, these cables (which generally comprise a stranded (or solid) conductor surrounded by a semi-conducting conductor shield, a polymeric insulation jacket, and an insulation shield), particularly those installed prior to 1985, often suffer premature breakdown and do not attain their originally anticipated longevity of 30 to 40 years. Their dielectric breakdown is generally attributed to so-called “treeing” phenomena (i.e., formation of microscopic voids or branching channels within the insulation material, from which the descriptive terminology derives), which lead to a progressive degradation of the cable's insulation. Since replacing a failed section of underground cable can be a very expensive and involved procedure, there is a strong motivation on the part of the electrical utility industry to extend the useful life of existing underground cables in a cost-effective manner.
A typical method for rejuvenating in-service cables comprises introducing a tree retardant fluid into the void space (interstitial void volume) associated with the strand conductor geometry. This fluid, which diffuses into the insulation and fills the microscopic trees to augment the service life of the cable, is generally selected from a particular class of aromatic alkoxysilanes, which can polymerize within the cable's interstitial void volume, as well as within the water tree voids in the insulation (Vincent et al. in U.S. Pat. No. 4,766,011). This method and variations thereof employing certain rapidly diffusing components (U.S. Pat. Nos. 5,372,840 and 5,372,841) have enjoyed commercial success over the last decade or so, but they still have some practical limitations when reclaiming underground residential distribution (URD) cables. The latter have a relatively small diameter (typically smaller than 4/0 and a conductor area of <107.2 mm2) and therefore present insufficient interstitial volume relative to the amount of retardant fluid required for optimum dielectric performance (e.g., sufficient retardant to saturate the conductor shield and insulation of the cable segment). This problem, however, is not limited to cables having such relatively small conductors since other geometric factors, such as strand compression and the greater insulation thickness associated with higher voltage cables, can also lead to insufficient interstitial volume in larger cables. Additionally, the specific properties of the treatment fluids (particularly solubility and diffusion) have an equally important influence. Thus, for all practical purposes, only a very small percentage of medium voltage or transmission voltage solid dielectric cables installed around the world do not suffer from the above mentioned inadequate interstitial volume relative to the amount of fluid required.
Therefore, although not explicitly required by the above mentioned disclosures, an in-the-field reclamation of URD cables employing the silane-based tree retardants typically leaves a fluid reservoir connected to the cable for a 60 to 90 day “soak period” to allow the tree retardant fluid to penetrate (i.e., diffuse into) the cable insulation and thereby restore the dielectric properties. As a result, it is generally necessary to have a crew visit the site at least three times: first, to begin the injection, which often involves a receiving bottle with an applied vacuum at one end and a slightly pressurized feed reservoir at the other end of the cable; second, to remove the receiving bottle a few days later after the fluid has traversed the length of the cable; and, finally, to remove the reservoir after the soak period is complete.
In detail, the current practice for restoring cables smaller than 4/0 or 107.2 mm2 having a stranded conductor comprises the following steps for a typical cable segment in a loop configuration:                (a) To avoid interruption of electrical power to utility customers, it is generally necessary to close the normally-open point in the circuit or loop. This step requires extensive coordination with the facility owner for safety considerations and requires repeated locking and unlocking of safety enclosures.        (b) Access the cable ends at an enclosure (typically a transformer or switch and sometimes a pole-mounted switch) and, using methods well know in the art, switch, de-energize, test for a de-energized condition with a voltmeter and ground the subject cable segment.        (c) Remove dead-front terminations and test the segment with a time domain reflectometer (TDR) to identify approximate locations of neutral corrosion and splices. If no significant corrosion is found, proceed to the next step;        if there is corrosion, abandon segment, either temporarily or permanently. See step (m).        
(d) Install injection terminations of the types disclosed in, e.g., U.S. Pat. Nos. 4,946,393, 5,082,449 or 6,332,785 or live-front injection adaptors well know in the art.                (e) Perform a gas flow test to identify any blockage or leak in splices using methods well known in the art. An electronic version of those well know methods is described in, e.g., U.S. Pat. No. 5,279,147. If not found, proceed to the next step; if there is blockage or leaking, abandon segment either temporarily or permanently. See step (m).        (f) Use a vacuum to evacuate most of the air from the cable. A vacuum is critical to this prior art approach since it typically represents about one-third of the available driving force of the injection. Further, not using vacuum results in residual bubbles in splices or other discontinuities along the flow path. Bubbles, of course, lead to regions of under-treatment, a huge issue when one considers that this low pressure approach typically under treats URD cables even under the best of circumstances.        (g) Inject desiccant (e.g., an anhydrous alcohol such as isopropyl alcohol or a mixture of anhydrous alcohols and alkoxy-functional silanes) into the interstitial void volume. This is believed to help flush excess water out of the cable strands, leaving the methoxy functionality of the primary treatment fluid to react with water in the cable's strand-shield and insulation system. The desiccation step improves safety of the subsequent injection while the cable is energized, as conventionally practiced, since the trapped water typically contains ionic contaminants and is a particularly good conductor. Additionally, even low pressure gas is a decent conductor (i.e., Paschen's Law) and displacement of air or nitrogen with organic and silane vapors also increases safety since the latter compounds have superior dielectric properties compared to the gases. Finally, the desiccant mitigates premature reaction of water with the treatment fluid, which would increase the bulk viscosity of the latter and impair efficient flow through the interstices.        (h) Inject tree retardant fluid using a pressure of less than 30 psig (pounds per square inch gage), leaving the cable unattended until the interstitial void volume is filled (several hours, and more typically, several days). In some cases, higher pressures may be employed, in which case the fluid will generally traverse the length of the segment and flush the strands while the injection crew is standing by. In this case, temporary rubber hoses and clamps facilitate injection but these are replaced with permanent low pressure terminations for the soak phase.        (i) Re-energize the cable segment (it is also possible to re-energize after step (d) and up to this point, but all following steps through (l) are carried out on energized equipment). Once this cable segment is re-energized, another cable segment in the loop may be switched out and treated by independently following steps (b) through (l), or the loop can be returned to its normal operational mode by opening the normally-open point.        (j) Reopen the transformer enclosure at the vacuum end of the cable segment to confirm sufficient fluid flush into the vacuum tank. If the vacuum is diminished, refresh the vacuum. Repeat this step until sufficient fluid flush has accumulated.        (k) Open the transformer enclosure at the feed end of the cable segment and confirm there is sufficient fluid for the soak phase. Generally this is only performed once, but for longer runs it may be necessary to repeat this step several times until the end of the designated soak period or until sufficient fluid has been introduced.        (l) Reopen the transformer enclosure at the feed end of the cable segment when the soak period is complete and remove the feed tank. Close and lock the enclosure.        
In addition, the following steps are required when a decision is made to address blocked or leaking splices and corroded neutrals, as mentioned in steps (c) and (e) above:                (m) Perform a cost-benefit analysis described by the “Repair Viability” graph and the associated text in the paper “Advancements in Cable Rejuvenation Technology” presented by Glen J. Bertini at IEEE/PES 1999 Summer Meeting, Reliability Centered Maintenance (Jul. 21, 1999) to determine if the incremental benefit of repair is greater than the incremental cost to repair. If the cost is greater than the benefit, abandon the effort; otherwise proceed to the next step.        (n) Schedule a digging crew/digging equipment to visit the site at the same time as the injection crew, previously having notified the governmental “No-dig” authority. The digging crew equipment may include some or all of the following: (1) shovels, (2) backhoes/excavators, or (3) vacuum excavators.        (o) Close the normally-open point in the circuit or loop.        (p) Access the cable ends at an enclosure and de-energize, test for a de-energized condition with a voltmeter, and ground the subject cable segment.        (q) Remove dead-front terminations and test the segment with a TDR to identify precise distances of neutral corrosion and splices from the cable ends.        (r) Attach an RF transmitter (e.g., Radio Detection RD 4000) to impress a radio tone on the cable and determine the precise location of the splice(s) and/or corrosion location(s).        (s) Dig the pit(s).        (t) Replace the blocked or leaking splices and/or repair the corrosion.        (u) Replace the excavated soils and repair any damage to landscaping or pavement.        (v) Return to step (d), above, and repeat the subsequent injection steps though step (l).        
Those skilled in the art will readily appreciate that it is not practical to de-energize a given cable segment each time it is visited since the switching process is time consuming (and thus expensive) and, when circuits are not in a loop configuration, the electrical end-user cannot be bothered with repeated outages. Thus, even though the circuit owner may desire to treat essentially all segments, the current practice is to leave a large percentage (typically 10 to 40%) untreated. To meet the circuit owner's reliability requirements, these untreated cables are typically replaced at a cost two to three times higher than that of restoration. Ironically, these cables are even more expensive to deal with than if they had been simply replaced to begin with because they were visited by an injection crew first and the labor expense is ultimately absorbed by the circuit owner. Moreover, the repetitive trips to an injection site are not only costly in terms of human resource, but each exposure of workers to energized equipment presents additional risk of serious injury or fatality and it is clearly beneficial to minimize such interactions. Thus, in view of the above limitations, a circuit owner might find it economically equivalent, or even advantageous, to completely replace a cable once it has deteriorated rather than resort to the above restorative methods.
In all of the above-recited methods for treating in-service cables, the tree retardant fluid is injected into the stranded conductor cable under a relatively low pressure sufficient to facilitate filling the interstitial void volume (typically less than about 30 psig). And, although considerably higher pressures have been employed to this end, the pressure was discontinued after the cable was filled.